i switch my external filter off to do maintenance and when i turn it back on again the intake pipe spits of loads of particles and bits, is this normal?
If happening to me as well counts as normal, then yes it is. I think it's where the crud has ended up on the outlet side of the filter media. Nothing to stop it so it gets fired back into the tank
It depends how much you clean out the filter media, the more you clean it, the less gunk you get when the filter is turned back on, but you don't want to completely clean the media, as much of what resides there is beneficial bacteria.
I get this effect sometimes normally if i have had to remove the top of one of my externals or it's got moved around during maintenance. Just looks a little unsightly but shouldn't cause any issues.